1. Technical Field
This disclosure is directed toward petroleum refining. More specifically, this disclosure is directed toward an improved process and apparatus for reducing sulfur concentrations in both effluent and vapor streams. The disclosed apparatus and method is capable of simultaneously reducing sulfur concentrations in at least one effluent stream and at least one vapor stream to levels of about 0.5 wt ppm or less, as required by modern catalytic reforming equipment.
2. Description of the Related Art
Hydrocracking is the petroleum refining process where complex organic molecules such as kerogens or heavy hydrocarbons are broken down into smaller, simpler molecules such as light hydrocarbons in a catalytic process. Hydrocracking yields middle distillates such as diesel and kerosene, gasoline components such as naphtha, and liquefied petroleum gas (LPG).
Hydrodesulfurization (HDS), also known as hydrotreating, is a catalytic chemical process widely used to remove sulfur from various components during the refining process The removal of sulfur compounds, or more specifically sulfur hydrocarbon compounds such as mercaptans, is important to reduce sulfur dioxide emissions as well as to avoid the poisoning of noble metal catalysts (e.g. platinum, rhenium, etc.) used in downstream catalytic reforming units that upgrade the octane rating of naphtha streams. For example, heavy naphtha product resulting from hydrocracking and/or hydrotreating process is low in octane must be catalytically “reformed” to improve the octane. Further, the presence of sulfur compounds will poison the catalysts used in a reforming unit.
However, in both hydrocracking and hydrotreating units, recombinant sulfur compounds can be formed when the reactor effluent material contains hydrogen sulfide above a certain temperature. The result is the formation of mercaptans that affect the quality of certain products and which boil in the naphtha range.
A catalytic reforming process rearranges or restructures hydrocarbon molecules in the heavy naphtha feed stock as well as breaking some of the compounds into smaller compounds. The overall effect is that the product reformate gains hydrocarbons with more complex molecular shapes having higher octane values than hydrocarbons that comprise the heavy naphtha feed stock. The naphtha feed stock is considered to be “heavy” when it has hydrocarbons with more than six carbon atoms and a naphtha feed stock is considered to be “light” when it includes hydrocarbons of six or less carbon atoms.
Modern reformer technology is typically limited to sulfur concentrations of about 0.5 wt ppm or less because of its reliance upon noble metal catalyst such as platinum or rhenium on a silica-alumina support. The activity of the catalysts is reduced over time by the presence of sulfur compounds. Therefore, the sulfur content of naphtha streams must be reduced prior to entering a reforming unit.
One prior art solution involves the use of a hydrotreating catalyst operating at reduced temperature that can selectively convert mercaptans to hydrogen sulfide and sulfur-free hydrocarbons. The hydrotreating bed must be operated at temperatures high enough for the reactions to take place but low enough to prevent subsequent formation of recombinant sulfur compounds due to the presence of the generated hydrogen sulfide. As a result, the location of the hydrotreating bed in a hot separator has been developed. The reactor effluent material enters a hot separator and the liquid falls and leaves the bottom of the separator vessel. The vapor travels upwardly through the hydrotreating or “post treat” bed and the mercaptan compounds are converted.
Another development is the concept of co-processing. Co-processing involves introducing a feed stream different than the primary feed which is passed through the separator with the sole objective of removing sulfur compounds. Co-processing is typically used downstream of hydrocracking units where there is a need to treat reactor effluent products to remove recombinant sulfur. The co-feed combines with the hydrocracking reactor effluent material and passes through the post treat bed. This concept is appropriate for distillate material which remains in the liquid phase at the conditions of the post treat bed.
Therefore, there is a need for an improved method and apparatus for removing sulfur compounds from naphtha streams including heavy naphtha streams, light naphtha streams and effluent containing naphtha.